In general, dual shear blade assemblies are preferred by the tree harvesting industry because considerably less tearing and wood damage occurs to a tree trunk as it is being cut, and less force is required to urge the individually shorter and thinner cutting blades part way through the wood. Representative of these dual shear blade assemblies are U.S. Pat. No. 3,503,429 issued Mar. 31, 1970 to E. L. Whisler and U.S. Pat. No. 3,627,003 issued Dec. 14, 1971 to K. Q. Kessler et al. The former uses a pair of opposing shear blade arrangements which are pivotally mounted on the frame, and each of which is individually controlled by a separate hydraulic actuating cylinder. Undesirably, the second hydraulic cylinder adds to the overall cost of the tree harvester and increases its overall size which, for example, can decrease its ability to be manipulated into a group of closely spaced trees. On the other hand, the latter of the aforementioned patents includes a pair of shear blade arrangements which are cooperatively manipulated by a single hydraulic cylinder. Such construction, however, includes a floating-type cylinder mounting that disadvantageously contributes to unsymmetrical opening and closing movement of the opposing shear blade arrangements. In addition, with the scissors type closing action of the cutting edges, a V-shaped slot is formed which creates forces tending to longitudinally separate the tree harvester and the tree which is being cut, which subsequently results in alignment problems and increased damage to the tree as it is being processed.
Other examples of prior art are disclosed in U.S. Pat. No. 3,509,922 issued May 5, 1970 to J. P. Lundberg and U.S. Pat. No. 3,565,141 issued Feb. 23, 1971 to A. J. Galis. However, the former construction incorporates an undesirably complex mechanism for simultaneously shearing a tree with three blades, and the tree must be inserted within a circular hole or sleeve during the processing thereof. While the latter construction, in contrast, is able to conveniently shear a standing tree, its three-blade construction is also relatively complex and costly, and incorporates one sliding blade with accompanying bearing support disadvantages.
As far as is known, not before the dual shear blade assembly disclosed in U.S. patent application Ser. No. 550,329 filed Feb. 18, 1975 by L. A. Wirt, now U.S. Pat. No. 3,927,704 and assigned to the assignee of the present invention, have many of the aforementioned problems been overcome. The reference construction can relatively more cleanly sever a tree because the pivotal connections for the dual shear blade arrangements are positioned on the frame in such a way that an improved mechanical advantage exists, and the tree is positively biased toward a fully retained position within the tree receiving slot by the penetrating cutting engagement of the shear blade arrangements. While such construction is a considerable advancement over the prior art, a second actuating cylinder is utilized therein and the tree harvester frame is larger than desired in order to accommodate it.